1. Field of the Invention
The present invention relates to a microphone output connector, and more specifically, to a technique for protecting a microphone case against invasion by a high-frequency electromagnetic wave generated by a cellular phone or the like, through the output connector. The present invention is advantageously used for, for example, a capacitor microphone output connector.
2. Description of the Related Art
Capacitor microphones incorporate an impedance converter such as a field effect transistor (FET) for very high impedance of a microphone unit. The capacitor microphones generally use a phantom power source. Microphone sound signals are output via a balanced shielded cable for the phantom power source.
A microphone case (a microphone grip in a case of a handled microphone) is installed with a three-pin type output connector for a connection with the balanced shielded cable (see, for example Japanese Patent Application Publication No. 2005-311752). FIG. 5 is a front view of an output connecter according to Japanese Patent Application Publication No. 2005-311752 extracted from a microphone case. FIG. 6 is a cross sectional view of the output connector installed in the microphone case.
According to the invention disclosed in Japanese Patent Application Publication No. 2005-311752, this output connector 10 includes a disk-shaped connector base 11 made of an electric insulator such as a polybutadiene terephthalate (PBT) resin. The connector base 11 has three pins, namely a first pin E of earthing, a second pin SH on a hot signal side, and a third pin SC on a cold signal side and the three pins are penetratingly installed by press fitting to the connector base 11.
In the case of the handheld microphone, the microphone connector 10 is installed in a cylindrical connector housing 20 screwed on an end of a microphone grip as shown in FIG. 6. The microphone grip and the cylindrical connector housing 20 are made of metal such as brass and also function as a shield case for electric parts incorporated therein.
When an intense electromagnetic wave is applied to a microphone or a microphone cable while the microphone cable (balanced shielded cable) connected out of a phantom power source (not shown) is connected to the output connector 10, the electromagnetic wave may enter the microphone through the output connector 10. In this case, the electromagnetic wave is demodulated by an impedance converter and output by the microphone as noise of an audible frequency.
A conventional technique known before the invention disclosed in Japanese Patent Application Publication No. 2005-311752, can effectively inhibit invasion by normal broadcasting waves, for example, electromagnetic waves of HF, VHF, UHF, or the like. However, the recent prevalence of cellular phones or the like has increased opportunities to use electromagnetic waves of higher frequencies near the microphone.
In the invention disclosed in Japanese Patent Application Publication No. 2005-311752, a male thread 12 is installed in the connector base 11 to electrically connect the first pin E for earthing to the cylindrical connector housing 20. The male thread 12 is housed in a thread housing hole 13 drilled in the connector base 11 in a radial direction. As shown in FIG. 6, a screwdriver (not shown) is inserted through a round hole 21 drilled in the cylindrical connector housing 20. The male thread 12 is rotated and pulled out using the screwdriver from the connector base 11. Thus, the male thread 12 is touched against the periphery of the round hole 21. A print circuit board 100 and a shield cover 200 are installed on an inner surface (which faces the interior of the microphone) side of the connector base 11 in order to protect the interior of the microphone (the interior of the microphone case) against the invasion by electromagnetic waves.
A shield layer of the print circuit board 100 electromagnetically shields the areas between three pins penetratingly installed in the connector base 11. On a top surface (part mounted surface) of the print circuit board 100: a capacitor element that prevents invasion by high-frequency wave; and a Zener diode element that protects the circuit from being electrostatically destroyed (both of which are not shown in the figure) are mounted in parallel. The surface on which these parts are mounted is covered with the shield cover 200, therefore, the microphone is protected against radiation by high frequency waves.
In the invention disclosed in Japanese Patent Application Publication No. 2005-311752, the shield cover 200 is in contact with the microphone case to protect the connector base 11 against invasion by electromagnetic waves. However, at only one point of a side opposite to the position of the connector base 11 at which the male thread 12 is screwed in, an outer periphery of the connector base 11 and an inner periphery of the cylindrical connector housing 20 are in line contact via the shield cover 200. Thus, the shield cover 200 and the cylindrical connector housing 20 are not in stable contact. On the side at which the male thread 12 is screwed in the connector base 11, a part of the shield cover 200 may be pressed against the inner surface of the cylindrical connector housing 20 with a flange (a shoulder) of the male thread 12. The part of the shield cover 200 may be pressed against the inner surface of the cylindrical connector housing 20 with the flange (the shoulder) of the male thread 12. Unfortunately, in this case, the shield cover 200 may be partly turned up to make the contact between the shield cover 200 and the cylindrical connector housing 20 less stable. Therefore, the invention disclosed in Japanese Patent Application Publication No. 2005-311752, which can provide the shielding effect, requires improvement. Moreover, a slight movement of the connector portion as a result of repeated putting on and pulling out of the microphone cord makes the contact less stable. With the less stable contact therebetween, a high frequency wave may not be blocked.